1. Field of the Invention
The present invention relates to a Wireless Local Area Network (WLAN) and a method of transmitting frames in the WLAN.
2. Description of the Related Art
A data transfer rate of the Wireless Local Area Network (WLAN) has been increased for years. A throughput of a cellular network is not high enough for multi-media service, however the WLAN provide 54 Mbps of Internet access service. The WLAN can be applicable to a mobile communication because the WLAN has wide bandwidth and low service charge.
IEEE 802.11 is an early wireless network standard for the WLAN. IEEE 802.11 compliant device operates at a maximum data transfer rate of 2 Mbps, transfers data frame using an infrared ray or a signal having 2.4 GHz Industrial, Scientific, Medical (ISM) band, and uses Carrier Sensing Multiple Access/Collision Avoidance (CSMA/CA) scheme.
IEEE 802.11b has emerged from IEEE 802.11 and provides a maximum data transfer rate of 11 Mbps. However, it provides a data transfer rate in a range of from 6 Mbps to 7 Mbps using the CSMA/CA scheme. IEEE 802.11a compliant devices operate at 5 GHz band. IEEE 802.11a can support a maximum data transfer rate of 54 Mbps using an Orthogonal Frequency Division Multiplexing (OFDM) scheme. Interference with other communication devices, such as mobile phones and Bluetooth devices in the 5 GHz band, is less than that in the 2.4 GHz, and the frequency band of 5 GHz is wider than that of 2 GHz. However, a 5 GHz band signal is easily affected by circumstances, such as obstacles and buildings. Thus, IEEE 802.11a is not widely used since the appearance of IEEE 802.11g that supports 54 Mbps of data transfer rate.
IEEE 802.11g has the same data transfer method and the same data transfer rate as those of IEEE 802.11a, and differs from IEEE 802.11a in that IEEE 802.11g uses a 2.4 GHz band signal. IEEE 802.11g is compatible with IEEE 802.11b. For example, IEEE 802.11g uses a frequency band in the range of from 2401 MHz to 2483 MHz. The full frequency band of IEEE 802.11g is divided into 113 frequency bands to which a total of 13 channels are assigned. A center frequency of the 13 channels begins at 2412 MHz, and center frequencies of the 13 channels are separated by 5 MHz. One channel occupies about a 22 MHz bandwidth, and thus interference occurs between channels having center frequencies which are not separated enough.
In a WLAN, some problems that do not occurs in a wire LAN are generated due to the cell coverage of the WLAN. A node ‘A’ and node ‘B’ cannot detect a signal from the other nodes due to the limited cell coverage. When nodes ‘A’ and ‘C’ respectively transmit data frame at the same time to node ‘B’, there is a high possibility of collision between the data frames of the nodes ‘A’ and ‘C’.
In a WLAN that transmits RTS/CTS frames, two additional frames, i.e. a Request To Send (RTS) frame and a Clear To Send (CTS) frame, are added to the IEEE 802.11 MAC frame so as to solve the problem noted above. When a source node transmits the RTS frame, the destination node transmits the CTS frame in response to the RTS frame. The RTS frame is transmitted before a data frame is transmitted. The nodes receiving the RTS and CTS frames cease to transmit frames during an interval that is contained in the RTS/CTS frames.
When the source node is ‘A’ and the destination node is ‘B’, the node ‘B’ that received the RTS frame from the node ‘A’ transmits the CTS frame to node ‘A’ in response to the RTS frame, and node ‘C’ can receive the CTS frame transmitted from the node ‘B’ since the node ‘C’ is located within the cell coverage of the node ‘B’. The node ‘C’ ceases to transmit frames during the interval that is contained in the CTS frame that is transmitted from the node ‘B’. Due to the cell coverage of the node ‘A’, nodes other than node ‘A’ cease to transmit frames for a predetermined interval from a time point when the node ‘A’ transmits the RTS frame. Due to the cell coverage of the node ‘B’, nodes other than node ‘B’ cease to transmit frames for a predetermined interval from a time point when the node ‘B’ transmits the CTS frame.
The RTS/CTS frame cannot be used when a large bandwidth is not required, or when nodes located in a small area can transmit/receive frames to/from each other, or when many channels are not concentrated.
In a virtual carrier sensing method of an IEEE 802.11 WLAN, in order to provide a reservation status of a channel, a node at a transmitter side transmits an RTS frame and a node at a receiver side transmits a CTS frame to the node at the transmitter side and nodes around the node at the receiver side in response to the RTS frame before a node transmits a data frame, or a reservation status of a channel can be provided using Duration/ID fields contained in the frame to be transmitted.
A method of providing a reservation status of a channel using the Duration/ID fields contained in the frame to be transmitted, and the other nodes are provided with an indication that a medium is busy (or reserved) during a predetermined interval using the Duration/ID fields. In the IEEE 802.11 WLAN, a node (sender) at a transmitter side transmits a frame having a Duration field containing information about an interval (duration) during which a node (receiver) at a receiver side can transmit an ACK frame to the sender without collision. The other nodes except the sender and the receiver cease to transmit a data frame (DATA) during the duration so that the receiver cam successfully transmit the ACK frame to the sender.
However, if a node does not provide the duration contained in the data frame transmitted from another node and the node transmits a data frame to another node, the ACK frame cannot be successfully transmitted.
Particularly, in a WLAN environment in which both wireless terminals conforming to IEEE 802.11b and wireless terminals conforming to IEEE 802.11g are located, it is impossible for a wireless terminal conforming to IEEE 802.11b to demodulate a data frame conforming to IEEE 802.11g using the OFDM modulation scheme, and thus a collision during data frame transmission can occur.